Sensor and image pickup device

ABSTRACT

A sensor for detecting a received electromagnetic wave comprising a first electrode, a second electrode and an amorphous oxide layer interposed between the first electrode and the second electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sensor for detecting a receivedelectromagnetic wave such as an optical sensor, a solar cell, or anX-ray sensor.

The present invention also relates to a non-flat image pickup device.

2. Related Background Art

The development of a thin film transistor (TFT) using an oxidesemiconductor thin film containing ZnO has been vigorously conducted(Japanese Patent Application Laid-Open No. 2003-298062).

The thin film can be formed at a low temperature, and is transparentwith respect to visible light. Accordingly, a flexible and transparentTFT can be formed on a substrate such as a plastic plate or a film. Inaddition, attempts have been made to use ZnO for an optical sensor and asolar cell.

Meanwhile, there are tubes that are complicatedly laid in an atomicpower plant or the like.

In addition, much cost and time have been spent for inspecting thecorroded states and the like of the tubes. Therefore, a non-flat X-rayimager (image pickup device) that can be inserted into a gap between thecomplicated tubes has been desired.

In the medical field, at present, a large burden has been applied to apatient in X-ray diagnosis by means of mammography or the like. Anon-flat X-ray imager as means for X-ray diagnosis imposing a reducedburden to a patient has been desired.

A non-flat imager is generally constituted by a thin film transistor andan X-ray sensor. The thin film transistor (TFT) is a three-terminaldevice equipped with a gate terminal, a source terminal, and a drainterminal. The TFT is combined with a sensor to be used as a switch forselecting a sensor or as an amplifier.

A more flexible one having better performance has been requested as asensor for detecting an electromagnetic wave or a non-flat X-ray imagepickup device.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a novel sensor or imagepickup device using an amorphous oxide.

Another object of the present invention is to provide a sensor ornon-flat image pickup device using an amorphous oxide having an electroncarrier concentration of less than 10¹⁸/cm³ or an amorphous oxide whoseelectron mobility tends to increase with increasing electron carrierconcentration.

Another object of the present invention is to provide an image pickupdevice comprising an X-ray sensor and a normally-off field effecttransistor.

The sensors for detecting a received electromagnetic wave in the presentinvention includes, of course, an optical sensor, and a sensor fordetecting non-visible light such as an ultraviolet optical sensor and asensor for detecting a radiant ray such as an X-ray sensor.

Hereinafter, the present invention will be described specifically.

According to one aspect of the present invention, there is provided asensor for detecting a received electromagnetic wave, including a firstelectrode; a second electrode; and an amorphous oxide layer interposedbetween the first electrode and the second electrode.

It is desirable that the amorphous oxide layer has an electron carrierconcentration of less than 10¹⁸/cm³.

The first electrode desirably has transmissivity with respect to lightin a wavelength range to which the amorphous oxide layer is sensitive.

The present invention also includes a sensor in which the amorphousoxide layer has an organic pigment.

According to another aspect of the present invention, there is provideda sensor including a first electrode; a second electrode; and anamorphous oxide semiconductor layer interposed between the firstelectrode and the second electrode, in which the amorphous oxide layeris an amorphous oxide whose electron mobility tends to increase withincreasing electron carrier concentration.

According to another aspect of the present invention, there is providedan image pickup device including:

a flexible substrate;

an X-ray sensor arranged on the flexible substrate; and

a field effect transistor electrically connected to the X-ray sensor, inwhich:

the field effect transistor has an amorphous oxide semiconductor as anactive layer; and

the amorphous oxide semiconductor has an electron carrier concentrationof less than 10¹⁸/cm³ or is an oxide whose electron mobility tends toincrease with increasing electron carrier concentration.

In particular, the image pickup device more preferably has a non-flatimaging region.

The present invention also includes a non-flat imager in which the X-raysensor includes a scintillator for converting X-ray into light and anopto-electric conversion element.

The present invention also includes an image pickup device, wherein theX-ray sensor comprises a semiconductor layer, and the semiconductorlayer also comprises an amorphous oxide.

According to another aspect of the present invention, there is providedan image pickup device comprising:

a substrate having a non-flat region;

an X-ray sensor provided on the substrate; and

a field effect transistor for reading a signal from the X-ray sensor,

wherein the field effect transistor is a normally-off transistor havingan active layer composed of an amorphous oxide.

By the way, in general, an inorganic thin film transistor is formed on aflat surface and is used in a flat shape. A conventional inorganic thinfilm transistor typified by amorphous silicon requires ahigh-temperature process for its formation, and it has been difficult toform such transistor on a flexible substrate such as a plastic resin.

Investigation has been made into a thin film transistor using an organicsemiconductor such as pentacene as a thin film transistor that can beformed on a flexible substrate. However, the characteristics of thetransistor have not been sufficient yet.

Recently, as described above, the development of a TFT using apolycrystalline oxide of ZnO for a channel layer has been vigorouslyconducted.

The inventors of the present invention have made investigation into anoxide semiconductor. As a result, they have found that ZnO cannotgenerally form a stable amorphous phase. In addition, most ZnO shows apolycrystalline phase. Therefore, a carrier is scattered at an interfacebetween polycrystalline particles, with the result that anelectron-mobility cannot be increased.

In addition, an oxygen defect is apt to enter ZnO. As a result, a largenumber of carrier electrons are generated, so it is difficult to reducean electric conductivity. It has been found that, owing to theforegoing, even when no gate voltage is applied to a transistor, a largecurrent flows between a source terminal and a drain terminal, so anormally-off operation of a TFT cannot be realized. It seems alsodifficult to increase on-off ratio of the transistor.

In addition, the inventors of the present invention have examined anamorphous oxide film Zn_(x)M_(y)In_(z)O_((x+3y/2+3z/2)) (where Mrepresents at least one element of Al and Ga) described in JapanesePatent Application Laid-Open No. 2000-044236. The material has anelectron carrier concentration of 10¹⁸/cm³ or more, so it is suitablefor a mere transparent electrode.

However, it has been found that, when an oxide semiconductor having anelectron carrier concentration of 10¹⁸/cm³ or more is used for a channellayer of a TFT, sufficient on-off ratio cannot be secured, so the oxideis not appropriate for a normally-off TFT.

That is, a conventional amorphous oxide film has been unable to providea film having an electron carrier concentration of less than 10¹⁸/cm³.

In view of the foregoing, the inventors of the present invention haveproduced a TFT using an amorphous oxide having an electron carrierconcentration of less than 10¹⁸/cm³ for an active layer of a fieldeffect transistor. As a result, they have obtained a TFT having desiredcharacteristics, and have discovered that the TFT is applicable to animage display device such as a light-emitting device.

The inventors of the present invention have conducted vigorous researchand development concerning InGaO₃(ZnO)_(m) and conditions under whichthe material is formed into a film. As a result, they have found that anelectron carrier concentration of less than 10¹⁸/cm³ can be achieved bycontrolling the conditions of an oxygen atmosphere upon film formation.

The present invention relates to a sensor or image pickup device using afilm that has realized a desired electron carrier concentration.

According to the present invention, there are provided a novel sensorand a novel image pickup device.

In particular, when a measuring object is subjected to X-raytransmittance measurement by means of a non-flat imager, an image havingreduced distortion as compared to a flat imager can be obtained.

In addition, when a human body is subjected to X-ray measurement, aphysical burden imposed on the measuring person is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relationship between the electron carrierconcentration of an In—Ga—Zn—O-based amorphous oxide semiconductorformed by means of a pulse laser deposition method and an oxygen partialpressure during film formation;

FIG. 2 is a graph showing a relationship between the electron carrierconcentration and electron mobility of an In—Ga—Zn—O-based amorphousoxide semiconductor formed by means of a pulse laser deposition method;

FIG. 3 is a graph showing a relationship between the electricconductivity of an In—Ga—Zn—O-based amorphous oxide semiconductor formedby means of a sputtering method using an argon gas and an oxygen partialpressure during film formation;

FIG. 4 shows graphs showing changes in electric conductivity, carrierconcentration, and electron mobility with the value for x ofInGaO₃(Zn_(1-x)Mg_(x)O) formed into a film by means of a pulse laserdeposition method in an atmosphere having an oxygen partial pressure of0.8 Pa;

FIG. 5 is a schematic structural view of a TFT produced for evaluatingan amorphous oxide semiconductor used for an optical sensor of thepresent invention;

FIG. 6 is a graph showing the current-voltage characteristics of a topgate MISFET device;

FIG. 7 is a graph showing the transmittance of an amorphoussemiconductor layer (200 nm) constituted by In—Ga—Zn—O;

FIG. 8 is a schematic explanatory view showing a first example of theoptical sensor of the present invention;

FIG. 9 is a second schematic explanatory view showing a second exampleof the optical sensor of the present invention;

FIG. 10 is a schematic structural view of an X-ray sensor of the presentinvention;

FIG. 11 is a pixel circuit diagram of a non-flat imager of the presentinvention;

FIG. 12 is a schematic explanatory view for explaining a method ofproducing the non-flat imager of the present invention;

FIG. 13 is a schematic explanatory view of X-ray measurement by means ofthe non-flat imager of the present invention;

FIG. 14 is a schematic view showing a pulse laser deposition apparatus;and

FIG. 15 is a schematic view showing a sputter film forming apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a sensor and a non-flat imager eachusing an amorphous oxide. Hereinafter, an optical sensor will bedescribed in detail in a first embodiment, and then a non-flat imagerwill be described in a second embodiment.

After that, an amorphous oxide common to both the embodiments and theproperties of the amorphous oxide will be described in detail.

First Embodiment

FIG. 8 shows a first schematic structural view of a sensor for detectinga received electromagnetic wave according to the present invention.

The sensor according to the present invention is constituted of a lowerelectrode (702), an amorphous oxide semiconductor layer (703), and anupper electrode (704) on a substrate (701).

The upper electrode may be referred to as the first electrode, and thelower electrode may be referred to as the second electrode.

In this embodiment, for example, an oxide having an electron carrierconcentration of less than 10¹⁸/cm³ is used for the amorphous oxidelayer.

The thickness of the amorphous oxide semiconductor layer, which isappropriately optimized depending on the wavelength of light with whichthe layer is irradiated or a pigment for pigment sensitization, ispreferably 10 nm to 1·m, or more preferably 10 nm to 500 nm.

The amorphous oxide semiconductor shows n-type conduction when it is asemiconductor containing In—Ga—Zn—O. It is also preferable to form ajunction between the amorphous oxide semiconductor and a metal having alarge work function such as Pt to constitute a photodiode. The oxidesemiconductor as an n-type oxide semiconductor and SrCu₂O₂ as a p-typeoxide semiconductor may be laminated to form a semiconductor junction,thereby constituting a photodiode.

Of course, the sensor according to the present invention can be used asan optical sensor for ultraviolet light or for X-rays. In addition, thesensor can be used as an optical sensor for visible light when it usesan organic pigment to be described later.

FIG. 9 shows a second schematic structural view of the sensor in thepresent invention.

The second sensor of the present invention is constituted of a lowerelectrode (802), a semiconductor layer (803) having a multilayerstructure which is constituted by laminating multiple amorphous oxidesemiconductor layers, and an upper electrode (804) on a substrate (801).

It is also preferable to form the semiconductor layer having amultilayer structure by appropriately optimizing the thickness of eachsemiconductor layer depending on the wavelength of light with which thelayer is irradiated. The thickness of each semiconductor layerconstituting the semiconductor layer having a multilayer structure ispreferably 1 nm to 100 nm, or more preferably 5 nm to 50 nm. The entirethickness of the semiconductor layer having a multilayer structure ispreferably 10 nm to 1·m, or more preferably 10 nm to 500 nm.

The semiconductor layer having a multilayer structure is constituted of,for example, amorphous oxide layers composed of mutually differentmaterials, or amorphous oxide layers having mutually differentthicknesses.

When light is applied from the upper electrode, a material and athickness through each of which applied light can transmit needs to beselected for the upper electrode. For example, an oxide transparentconductive film is preferable. When light is applied from the side ofthe substrate, a quartz material, an acrylic resin, or the like whichare excellent in translucency is a preferable material for thesubstrate. In this case, an oxide transparent conductive film having awide band gap is preferably used for the lower electrode.

The case where the amorphous oxide semiconductor of the presentinvention is sensitized with an organic pigment will be described.

As shown in FIG. 8, in the case where light incidence takes place froman upper portion of the optical sensor device, the amorphous oxidesemiconductor film is deposited and then immersed in an organic solventinto which an organic pigment is dissolved to cause the organic pigmentto adsorb to the semiconductor. Alternatively, the organic pigment isdeposited from the vapor onto the semiconductor by means of a vacuumdeposition method. After that, the upper electrode is formed by means ofa vacuum deposition method or a sputtering method.

In the case where light incidence takes place from the side of thesubstrate, the organic pigment is caused to adsorb to the lowerelectrode. After that, the amorphous oxide semiconductor is formed bymeans of a laser ablation method or a sputtering method.

Furthermore, as shown in FIG. 9, in the case where a semiconductor layerhaving a multilayer structure is to be sensitized with an organicpigment, the semiconductor layer having a multilayer structure can beformed by repeatedly laminating organic pigments by means of animmersion method, a deposition method, or the like every time eachsemiconductor layer is laminated.

In that case, it is preferable to make such a distribution as to changefrom a pigment capable of absorbing light having a short wavelength to apigment capable of absorbing light having a long wavelength as lightenters a deeper portion of the semiconductor layer from its incidenceside.

The substrate to be used in the present invention may be conductive orelectrically insulating properties. Examples of a conductive substrateinclude metals and alloys of the metals such as NiCr, stainless steel,Al, Cr, Mo, Au, Nb, Ta, V, Ti, Pt, and Pb. Examples of an electricallyinsulating substrate include films made of synthetic resins such as anacrylic resin, polyester, polyethylene, polycarbonate, celluloseacetate, polypropylene, polyvinyl chloride, polyvinylidene chloride,polystyrene, and polyamide; sheets; glass; ceramics; and paper. At leastone surface of each of those electrically insulating substrates ispreferably subjected to a conductive treatment, and a light-receivinglayer is desirably arranged on the surface subjected to the conductivetreatment.

For example, a thin film composed of NiCr, Al, Cr, Mo, Au, Ir, Nb, Ta,V, Ti, Pt, Pd, InO₃, ITO (In₂O₃+Sn) or the like is arranged on thesurface of glass to impart conductivity. Alternatively, a thin filmcomposed of a metal such as NiCr, Al, Ag, Pb, Zn, Ni, Au, Cr, Mo, Ir,Nb, Ta, V, Tl, or Pt is arranged on the surface of a synthetic resinfilm such as a polyester film by means of vacuum deposition, electronbeam deposition, sputtering, or the like, or the surface is subjected tolamination with the metal, to impart conductivity to the surface. Thesubstrate is preferably a substrate having flexibility, that is, thesubstrate is preferably deformable (especially bendable).

The light transmittance of an oxide transparent conductive film to beused in the present invention is preferably 60% or more, or morepreferably 85% or more. In addition, the film desirably has a sheetresistance of 100·or less so as not to serve as an electrical resistancecomponent with respect to the output of a photovoltaic device. The term“light transmittance” mentioned above refers to the transmittance oflight in a wavelength range to be detected by an optical sensor.Examples of a material having such properties include extremely thin andtransparent metal films formed of metal oxides such as SnO₂, In₂O, ITO(SnO₂+In₂O₃), ZnO, CdO, Cd₂SnO₄, TiO₂, and Ti₃N₄; and metals such as Au,Al, and Cu.

Of those, a transparent electrode made of an indium oxide or anindium-tin oxide is particularly suitable. Examples of an availablemethod of producing the electrode include a resistance heatingdeposition method, an electron beam heating deposition method, asputtering method, and a spray method, and the methods are appropriatelyselected as desired. Of those, a sputtering method and a vacuumdeposition method are optimum deposition methods.

The organic pigment is selected from a cyanine pigment, a merocyaninepigment, a phthalocyanine pigment, a naphthalocyanine pigment, aphthalo/naphthalo-mixed phthalocyanine pigment, a dipyridyl Ru complexpigment, a terpyridyl Ru complex pigment, a phenanthroline Ru complexpigment, a phenylxanthene pigment, a triphenylmethane pigment, acoumarin pigment, an acridine pigment, and an azo metal complex pigmenteach of which can chemically bond to the semiconductor. Of course,multiple pigments may be used in combination.

An organic pigment sensitizer suitable for the present invention ispreferably one capable of forming a bond with the amorphous oxidesemiconductor mainly composed of In—Ga—Zn—O of the present invention,the bond facilitating the movement of photo-excited charge.

A pigment that adsorbs to a semiconductor layer to function as aphotosensitizer is one showing absorption in various visible lightregions and/or an infrared light region.

A pigment preferably has, in a pigment molecule, a carboxylic group, acarboxylic anhydride group, an alkoxy group, a hydroxyalkyl group, asulfonic group, a hydroxyl group, an ester group, a mercapto group, aphosphonyl group, or the like in order to cause the pigment to stronglyadsorb to the semiconductor layer.

Of those, a carboxylic group and a carboxylic anhydride group areparticularly preferable. It should be noted that each of the groupsprovides an electrical bond that facilitates the movement of an electronbetween a pigment in an excited state and the conduction band of theamorphous oxide semiconductor.

Examples of pigments having the groups include a rutheniumbipyridine-based pigment, an azo-based pigment, a quinone-based pigment,a quinoneimine-based pigment, a quinacridone-based pigment, atriphenylmethane-based pigment, and a xanthene-based pigment. Theexamples further include a squarilium-based pigment, a cyanine-basedpigment, a merocyanine-based pigment, a porphyrin-based pigment, aphthalocyanine-based pigment, a perylene-based pigment, an indigo-basedpigment, and a naphthalocyanine-based pigment.

Examples of a method of causing the pigment to adsorb to thesemiconductor layer include a method involving immersing a semiconductorlayer formed on a conductive substrate into a solution into which apigment is dissolved (a solution for pigment adsorption); and a methodinvolving depositing an organic pigment from the vapor. The examplesfurther include a method involving heating an organic pigment,transporting the organic pigment by means of an inert gas such as heliumor nitrogen, and causing the organic pigment to adsorb to asemiconductor. It is preferable to form an organic pigment on anamorphous oxide semiconductor in a monomolecular layer fashion.

Any solvent can be used as long as it is capable of dissolving apigment, and specific examples of such solvent include alcohols such asethanol; ketones such as acetone; ethers such as diethyl ether andtetrahydrofuran; and nitrogen compounds such as acetonitrile. Theexamples further include halogenated aliphatic hydrocarbons such aschloroform; aliphatic hydrocarbons such as hexane; aromatic hydrocarbonssuch as benzene; esters such as ethyl acetate; and water. Two or morekinds of those solvents may be used as a mixture.

The pigment concentration in a solution, which can be appropriatelyadjusted depending on the pigment to be used and a kind of solvent, ispreferably as high as possible in order to enhance adsorption function.For example, the pigment concentration is preferably 1×10⁻⁵ mol/l ormore, or more preferably 1×10⁻⁴ mol/l or more.

In the present invention, an organic pigment corresponding to awavelength range as a target for an optical sensor device is preferablyselected and used in an appropriate manner. A single pigment may be usedfor the pigment, or multiple pigments may be used in combination for thepigment.

Second Embodiment

FIG. 10 shows a schematic view of an image pickup device according tothe present invention. The invention according to this embodiment is,for example, an X-ray image sensor. The image pickup device of thepresent invention is constituted of a lower electrode 902, asemiconductor layer 903 to serve as an opto-electric conversion element,an upper electrode 904, and a scintillator 905 on a deformable substrate901. The constitution shown in FIG. 8 or 9 can be used for theconstitution from the substrate 901 to the upper electrode 904. Thesemiconductor layer is formed of, for example, an amorphous oxidecontaining at least In—Ga—Zn—O.

At least part of the image pickup device according to the presentinvention preferably has a non-flat portion. Of course, an image pickupdevice which instantaneously has a flat shape but can be deformed into anon-flat shape is also preferable.

An amorphous oxide (to be described in detail later) may be used for thesemiconductor layer 903, or amorphous silicon or the like may be usedfor the layer. As described later, an oxide having an electron carrierconcentration of less than 10¹⁸/cm³ or an oxide whose electron mobilitytends to increase with increasing electron carrier concentration can beused for the amorphous oxide.

In addition to a glass substrate or the like, a resin, plastic, orpolyethylene terephthalate (PET) is applicable to the substrate. Thesubstrate is preferably a flexible substrate.

In the X-ray sensor, the scintillator 905 mainly using a phosphor isused as required, and may be omitted when the above-describedsemiconductor layer is sensitive to X-ray.

NaI (Tl) (deliquescent), CsI (Tl) (deliquescent), Cs (Na)(deliquescent), CsI (pure), CaF₂ (Eu), BaF₂, CdWO₄, or the like is usedfor the scintillator. The thickness of the scintillator layer ispreferably in the range of 100·m to 500·m because the thickness in therange allows the layer to sufficiently absorb X-ray. The scintillatorlayer is preferably formed by means of a sputtering method. Before adeliquescent scintillator is used, the scintillator needs to besubjected to a dampproofing treatment. A preferable dampproofingtreatment involves laminating a dampproofing layer (such as a siliconnitride layer or a silicon oxide layer) having a thickness of 100 nm ormore on each of the rear surface of the substrate 901 and the surface ofthe scintillator.

FIG. 11 shows a circuit per one pixel using a TFT using an amorphousoxide such as In—Ga—Zn—O according to the present invention for anactive layer; and an X-ray sensor composed of a scintillator and anopto-electric conversion element using an oxide semiconductor containingIn—Ga—Zn—O.

The TFT is preferably of a normally-off TFT in which the amorphousoxides as described later are used for an active layer of the TFT.

The sensing operation of an imaging sensor having such athree-transistor pixel structure is as follows.

In an X-ray sensor 1006, an X-ray enters a scintillator to be convertedinto visible light. The light is converted into electricity by anIn—Ga—Zn—O-containing oxide semiconductor sensitized with a pigment. Theconverted signal charge changes the potential of a floating node 1005 asa source end of a reset TFT 1001. As a result, the gate potential of aselect TFT 1004 as a driver for a pixel level source follower ischanged. The bias of the source end of the select TFT 1004 or of thedrain node of an access TFT 1007 is changed.

While signal charge is accumulated in this way, the potentials of thesource end of the reset TFT 1001 and the source end of the select TFT1004 change. At this time, if a low selection signal is inputted to thegate of the access TFT 1007 via a low selection signal input terminal1008, a potential difference due to the signal charge generated by theX-ray sensor 1006 will be outputted toward a column selection line 1009.

In this way, a signal level due to the generation of charge by the X-raysensor 1006 is detected. After that, a reset signal via a reset signalinput terminal 1002 changes a reset transistor 1 into an on state. Thus,the signal charge accumulated in the X-ray sensor 1006 is entirelyreset.

When a semiconductor layer which is sensitive to X-ray is used, ofcourse, the scintillator can be omitted. In addition, the organicpigment can be omitted. The active layer of the above TFT can be formedof, for example, an amorphous oxide to be described later. For example,an oxide having an electron carrier concentration of less than 10¹⁸/cm³or an oxide whose electron mobility tends to increase with increasingelectron carrier concentration can be used for the amorphous oxide. Theabove TFT can be arranged on one side of the opto-electric conversionelement with the aid of the upper electrode 904, the semiconductor layer903, and the lower electrode 902. A layer for the above TFT may beseparately arranged between the substrate 901 and the lower electrode902.

The thickness of the oxide semiconductor layer in respect of X-rayabsorption is 50·m or larger, preferably 100·m or larger, and morepreferably 300 ·m or larger.

Both the semiconductor of the X-ray sensor and the active layer of theTFT for receiving (or reading) a signal from the sensor can be comprisedof an amorphous oxide, which is a preferable constitution in the casewhere higher flatness is required.

Multiple imaging sensor units shown in FIG. 11 each formed on adeformable substrate were arranged to constitute a non-flat imager. Theresolution of the imager was set to be equal to or larger than XGA(1,024×768), SXGA (1,280×1,024), or the like. FIG. 12 shows an exampleof a method of producing the non-flat imager. As shown on the left sideof FIG. 12, sensor units produced on a flat surface are cut at brokenlines to constitute a spherical shape as shown on the right side of thefigure. Thus, a semispherical non-flat imager is constituted. Referencenumeral 1101 denotes a TFT and a sensor formed on a flat surface.Reference numeral 1102 denotes a TFT and a sensor unit. Referencenumeral 1103 denotes a semispherical non-flat imager provided with theTFT and the sensor described above.

The arrangement of a TFT and a sensor portion on the non-flat surface ofthe non-flat imager can be realized as follows. For example, at first, aTFT and the like are arranged on a flexible substrate made of plastic,PET, or the like, that is, on a flat surface. After that, the flexiblesubstrate is pressed against a non-flat mold while the substrate isheated, to thereby deform the flat substrate into a non-flat substrate.Of course, the term “non-flat imager” as used herein comprehends both animager having a flat region and a non-flat region and an imager that canbe deformed from a flat state to a non-flat state.

FIG. 13 shows an example of a measurement method by means of thenon-flat imager of the present invention. A measuring object 1203 is fedinto a non-flat imager 1201 formed in FIG. 12, and the resultant isirradiated with an external X-ray 1204 to subject the measuring object1203 to measurement. It is also preferable to form a sensor unit in onehalf of the semispherical non-flat imager in such a manner that nosensor unit enters a gap between an X-ray source and the measuringobject.

(Amorphous Oxide Semiconductor)

As described above, in the present invention, an amorphous oxidesemiconductor having a desired electron carrier concentration is used asan optical sensor portion itself or as an active layer of a field effecttransistor to be used for an optical sensor. Of course, the amorphousoxide semiconductor may be used as each of them.

The electron carrier concentration of the amorphous oxide semiconductoraccording to the present invention is a value measured at roomtemperature. Room temperature is, for example, 25° C., and,specifically, is a temperature appropriately selected from the range ofabout 0° C. to 40° C. It should be noted that there is no need for theelectron carrier concentration of the amorphous oxide semiconductoraccording to the present invention to have a value of less than 10¹⁸/cm³in the entire range of 0° C. to 40° C. For example, an electron carrierconcentration of less than 10¹⁸/cm³ has only to be realized at 25° C. Inaddition, reducing the electron carrier concentration to 10¹⁷/cm³ orless, or more preferably 10¹⁶/cm³ or less provides a normally-off TFTwith high yield.

The electron carrier concentration can be measured through Hall effectmeasurement.

The term “amorphous oxide” as used herein refers to an oxide having ahalo pattern to be observed, and showing no specific diffraction ray, inan X-ray diffraction spectrum.

The lower limit for the electron carrier concentration in the amorphousoxide semiconductor of the present invention is not particularly limitedas long as the amorphous oxide semiconductor is applicable to a channellayer of a TFT. The lower limit is, for example, 10¹²/cm³.

Therefore, in the present invention, as in each of the examples to bedescribed later, the electron carrier concentration is set to fallwithin the range of, for example, preferably 10¹²/cm³ (inclusive) to10¹⁸/cm (exclusive), more preferably 10¹³/cm³ to 10¹⁷/cm³ (bothinclusive), or still more preferably 10¹⁵/cm to 10¹⁶/cm (both inclusive)by controlling the material, composition ratio, production conditions,and the like of the amorphous oxide.

In addition to an InZnGa oxide, the amorphous oxide can be appropriatelyselected from an In oxide, an In_(x)Zn_(1-x) oxide (0.2·x·1), anIn_(x)Sn_(1-x) oxide (0.8·x·1), and an In_(x)(Zn, Sn)_(1-x) oxide(0.15·x·1).

The In_(x)(Zn, Sn)_(1-x) oxide can be described as anIn_(x)(Zn_(y)Sn_(1-y))_(1-x) oxide, and y ranges from 1 to 0.

Part of In in an In oxide containing none of Zn and Sn can be replacedwith Ga. That is, the In oxide can be turned into an In_(x)Ga_(1-x)oxide (0·x·1).

Hereinafter, an amorphous oxide having an electron carrier concentrationof less than 10¹⁸/cm³ that the inventors of the present invention havesucceeded in producing will be described in detail.

The oxide contains In—Ga—Zn—O, its composition in a crystalline state isrepresented by InGaO₃(ZnO)_(m) (where m represents a natural number ofless than 6), and its electron carrier concentration is less than10¹⁸/cm³.

The oxide contains In—Ga—Zn—Mg—O, its composition in a crystalline stateis represented by InGaO₃(Zn_(1-x)Mg_(x)O)_(m) (where m represents anatural number of less than 6 and 0<x·1), and its electron carrierconcentration is less than 10¹⁸/cm³.

A film constituted of each of those oxides is preferably designed tohave an electron mobility in excess of 1 cm²/(V·sec).

The use of the film for a channel layer enables transistorcharacteristics including a gate current at the time of turning atransistor off of less than 0.1·A (that is, normally off) and an on-offratio in excess of 10³. In addition, the use realizes a flexible TFT,which is transparent, or has transmissivity, with respect to visiblelight.

The electron mobility of the film increases with increasing number ofconduction electrons. A glass substrate, a plastic substrate made of aresin, a plastic film, or the like can be used as a substrate forforming a transparent film.

When the amorphous oxide semiconductor film is used for a channel layer,one of Al₂O₃, Y₂O₃, and HfO₂, or a mixed crystal compound containing atleast two kinds of these compounds can be used for a gate insulationfilm.

It is also preferable to form the amorphous oxide into a film in anatmosphere containing an oxygen gas without intentionally adding anyimpurity ion for increasing an electrical resistance of the oxide.

The inventors of the present invention have found that thesemi-insulating oxide amorphous thin film has specific property withwhich the electron mobility of the film increases with increasing numberof conduction electrons. Furthermore, the inventors have found that aTFT produced by means of the film is provided with additionally improvedtransistor characteristics including on-off ratio, saturation current ina pinch-off state, and switching speed. That is, the inventors havefound that a normally-off TFT can be realized by using an amorphousoxide.

The use of the amorphous oxide thin film for a channel layer of a filmtransistor provides an electron mobility in excess of 1 cm²/(V·sec),preferably in excess of 5 cm²/(V·sec).

When the electron carrier concentration is less than 10¹⁸/cm³, orpreferably less than 10¹⁶/cm³, a current between drain and sourceterminals at the time of off (when no gate voltage is applied) can beset to be less than 10·A, or preferably less than 0.1·A.

The use of the film provides saturation current after pinch-off inexcess of 10·A and an on-off ratio in excess of 10³ when the electronmobility exceeds 1 cm²/(V·sec), or preferably exceeds 5 cm²/(V·sec).

In a TFT, a high voltage is applied to a gate terminal in a pinch-offstate, and electrons are present in a channel at a high density.

Therefore, according to the present invention, saturation current valuecan be increased by an amount corresponding to an increase in electronmobility. As a result, improvements of transistor characteristicsincluding an increase in on-off ratio, an increase in saturationcurrent, and an increase in switching speed can be expected.

In a typical compound, when the number of electrons increases, electronmobility reduces owing to a collision between electrons.

Examples of a structure that can be used for the TFT include a stagger(top gate) structure in which a gate insulation film and a gate terminalare formed in order on a semiconductor channel layer; and an inverselystaggered (bottom gate) structure in which a gate insulation film and asemiconductor channel layer are formed in order on a gate terminal.

(First Film Forming Method: PLD Method)

The amorphous state of an amorphous oxide thin film whose composition ina crystalline state is represented by InGaO₃(ZnO)_(n) (where mrepresents a natural number of less than 6) is stably maintained up to ahigh temperature equal to or higher than 800° C. when the value of m isless than 6. However, as the value of m increases, that is, as the ratioof ZnO to InGaO₃ increases to cause the composition to be close to a ZnOcomposition, the thin film is apt to crystallize.

Therefore, a value of m of less than 6 is preferable for a channel layerof an amorphous TFT.

A vapor phase deposition method involving the use of a polycrystallinesintered material having an InGaO₃(ZnO)_(m) composition as a target is adesirable film forming method. Of such vapor phase deposition methods, asputtering method and a pulse laser deposition method are suitable.Furthermore, a sputtering method is most suitable from the viewpoint ofmass productivity.

However, when the amorphous film is produced under typical conditions,an oxygen defect mainly occurs, so it has been unable to provide anelectron carrier concentration of less than 10¹⁸/cm³, that is, anelectric conductivity of 10 S/cm or less. The use of such film makes itimpossible to constitute a normally-off transistor.

The inventors of the present invention have produced In—Ga—Zn—O by meansof a pulse laser deposition method with the aid of an apparatus shown inFIG. 14.

Film formation was performed by means of such PLD film forming apparatusas shown in FIG. 14.

In the figure, reference numeral 701 denotes a rotary pump (RP); 702, aturbo-molecular pump (TMP); 703, a preparatory chamber; 704, an electrongun for RHEED; 705, substrate holding means for rotating, and movingvertically, a substrate; 706, a laser entrance window; 707, thesubstrate; 708, a target; 709, a radical source; 710, a gas inlet; 711,target holding means for rotating, and moving vertically, the target;712, a bypass line; 713, a main line; 714, a turbo-molecular pump (TMP);715, a rotary pump (RP); 716, a titanium getter pump; and 717, ashutter. In addition, in the figure, reference numeral 718 denotes anion vacuum gauge (IG); 719, a Pirani vacuum gauge (PG); 720, a baratronvacuum gauge (BG); and 721, a growth chamber (chamber).

An In—Ga—Zn—O-based amorphous oxide semiconductor thin film wasdeposited on an SiO₂ glass substrate (1737 manufactured by Corning Inc.)by means of a pulse laser deposition method using a KrF excimer laser.Prior to the deposition, the substrate was subjected to degreasingwashing by means of an ultrasonic wave for 5 minutes in each of acetone,ethanol, and ultrapure water, and then was dried in the air at 100° C.An InGaO₃(ZnO)₄ sintered material target (having a diameter of 20 mm anda thickness of 5 mm) was used as the polycrystalline target. The targetwas produced by wet-mixing 4N reagents of In₂O₃, Ga₂O₃, and ZnO asstarting materials in ethanol as a solvent; calcining the mixture at1,000° C. for 2 hours; dry-pulverizing the resultant; and sintering thepulverized product at 1,550° C. for 2 hours. The target thus producedhad an electric conductivity of 90 (S/cm).

Film formation was performed with the ultimate pressure in the growthchamber set to 2×10⁻⁶ (Pa) and the oxygen partial pressure during growthcontrolled to be 6.5 (Pa).

The oxygen partial pressure in the chamber 721 was 6.5 Pa and thesubstrate temperature was 25° C.

The distance between the target 708 and the deposition substrate 707 was30 (mm), and the power of the KrF excimer laser incident from theentrance window 706 was in the range of 1.5 to 3 (mJ/cm²/pulse). Thepulse width, pulse rate, and irradiation spot diameter were set to 20(nsec), 10 (Hz), and 1×1 (mm square), respectively.

Thus, film formation was performed at a film-forming rate of 7 (nm/min).

X-ray diffraction was conducted on the resultant thin film by means ofan X-ray at an angle of incidence as close as the thin film (thin filmmethod, angle of incidence 0.5 degree). As a result, no cleardiffraction peak was observed. Therefore, the produced In—Ga—Zn—O-basedthin film can be said to be amorphous.

Furthermore, X-ray reflectance measurement was performed, and patternanalysis was performed. As a result, the thin film was found to have amean square roughness (Rrms) of about 0.5 nm and a thickness of about120 nm. X-ray fluorescence (XRF) analysis confirmed that the metalcomposition ratio of the thin film was In:Ga:Zn=0.98:1.02:4.

The film had an electric conductivity of less than about 10⁻² S/cm. Theelectron carrier concentration and electron mobility of the film areestimated to be about 10¹⁶/cm³ or less and about 5 cm²/(V·sec),respectively.

Owing to the analysis of a light absorption spectrum, the forbidden bandenergy width of the produced amorphous thin film was determined to beabout 3 eV. The foregoing shows that the produced In—Ga—Zn—O-based thinfilm is a transparent and flat thin film showing an amorphous phaseclose to the composition of InGaO₃(ZnO)₄ as a crystal, having littleoxygen defect, and having a small electric conductivity.

Specific description will be made with reference to FIG. 1. The figureshows change of the electron carrier concentration of a transparentamorphous oxide thin film formed with changing oxygen partial pressureunder the same conditions as those of this embodiment, which film iscomposed of In—Ga—Zn—O and has a composition in an assumed crystallinestate represented by InGaO₃(ZnO)_(m) (where m represents a number ofless than 6).

Film formation was performed in an atmosphere having a high oxygenpartial pressure in excess of 4.5 Pa under the same conditions as thoseof this embodiment. As a result, as shown in FIG. 1, it was able toreduce the electron carrier concentration to less than 10¹⁸/cm³. In thiscase, the substrate had a temperature maintained at a temperature nearlyequal to room temperature unless intentionally heated. The substratetemperature is preferably kept at a temperature lower than 100° C. inorder to use a flexible plastic film as a substrate.

Additionally increasing the oxygen partial pressure can additionallyreduce the electron carrier concentration. For example, as shown in FIG.1, an InGaO₃(ZnO)₄ thin film formed at a substrate temperature of 25° C.and an oxygen partial pressure of 5 Pa had a number of electron carriersreduced to 10¹⁶/cm³.

As shown in FIG. 2, the resultant thin film had an electron mobility inexcess of 1 cm²/(V·sec). However, in the pulse laser deposition methodof this embodiment, when the oxygen partial pressure is 6.5 Pa or more,the surface of the deposited film becomes irregular, so it becomesdifficult to use the film as a channel layer of a TFT.

Therefore, a normally-off transistor can be constituted by using atransparent amorphous oxide thin film having a composition in acrystalline state represented by InGaO₃(ZnO)_(m) (where m represents anumber of less than 6) by means of a pulse laser deposition method in anatmosphere having an oxygen partial pressure in excess of 4.5 Pa, ordesirably in excess of 5 Pa and less than 6.5 Pa.

In addition, the thin film had an electron mobility in excess of 1cm²/V·sec, so the on-off ratio was able to exceed 10³.

As described above, when an InGaZn oxide is formed into a film by meansof the PLD method under the conditions shown in this embodiment, theoxygen partial pressure is desirably controlled to be 4.5 Pa or more andless than 6.5 Pa.

The realization of an electron carrier concentration of less than10¹⁸/cm³ depends on, for example, a condition for an oxygen partialpressure, the structure of a film forming apparatus, and a material anda composition to be formed into a film.

Next, an amorphous oxide was produced at an oxygen partial pressure of6.5 Pa in the above apparatus, and then a top gate MISFET device shownin FIG. 5 was produced. To be specific, at first, a semi-insulatingamorphous InGaO₃(ZnO)₄ film having a thickness of 120 nm to be used as achannel layer 2 was formed on a glass substrate 1 by means of theabove-described method of producing an amorphous In—Ga—Zn—O thin film.

Then, InGaO₃(ZnO)₄ and a gold film each having a large electricconductivity and a thickness of 30 nm were laminated on the film bymeans of a pulse laser deposition method with the oxygen partialpressure in the chamber set to be less than 1 Pa, to thereby form adrain terminal 5 and a source terminal 6 by means of a photolithographymethod and a lift-off method. Finally, a Y₂O₃ film to be used as a gateinsulation film 3 (thickness: 90 nm, relative dielectric constant: about15, leak current density: 10⁻³ A/cm upon application of 0.5 MV/cm) wasformed by means of an electron beam deposition method. A gold film wasformed on the Y₂O₃ film, to thereby form a gate terminal 4 by means of aphotolithography method and a lift-off method.

Evaluation of MISFET Device for Characteristics

FIG. 6 shows the current-voltage characteristics of an MISFET devicemeasured at room temperature. The fact that a drain current I_(DS)increased with increasing drain voltage V_(DS) shows that the channel isan n-type semiconductor. This is not in contradiction to the fact thatan amorphous In—Ga—Zn—O-based semiconductor is of an n-type. I_(DS)saturated (pinched off) at V_(DS) of about 6 V. The saturation is atypical behavior of a semiconductor transistor. Investigation into again characteristic showed that the threshold value for a gate voltageV_(GS) was about −0.5 V upon application of V_(DS)=4 V. A currentI_(DS)=1.0×10⁻⁵ A flowed when V_(G)=10 V. This corresponds to the factthat a gate bias enabled a carrier to be induced in an In—Ga—Zn—O-basedamorphous semiconductor thin film as an insulator.

The transistor had an on-off ratio in excess of 10³. The field effectmobility was calculated from an output characteristic. As a result, afield effect mobility of about 7 cm² (Vs)⁻¹ was obtained in thesaturation region. The produced device was irradiated with visible lightto perform similar measurement. However, no changes in transistorcharacteristics were observed.

According to this embodiment, a thin film transistor having a channellayer with a small electron carrier concentration (that is, a highelectrical resistance) and a large electron mobility can be realized.

The above-described amorphous oxide had excellent properties. That is,electron mobility increased with increasing electron carrierconcentration, and degenerate conduction was exhibited.

In this embodiment, a thin film transistor was formed on a glasssubstrate. A substrate such as a plastic plate or a film can also beused because film formation itself can be performed at room temperature.

In addition, the amorphous oxide obtained in this embodiment absorbsnearly no visible light and can realize a transparent and flexible TFT.

(Second Film Forming Method: Sputtering Method (SP Method))

Description will be given of film formation by means of a high-frequencySP method using an argon gas as an atmospheric gas.

The SP method was performed by means of an apparatus shown in FIG. 15.In the figure, reference numeral 807 denotes a deposition substrate;808, a target; 805, substrate holding means equipped with a coolingmechanism; 814, a turbo-molecular pump; 815, a rotary pump; 817, ashutter; 818, an ion vacuum gauge; 819, a Pirani vacuum gauge; 821, agrowth chamber (chamber); and 830, a gate valve.

An SiO₂ glass substrate (1737 manufactured by Corning Inc.) was preparedas the deposition substrate 807. Prior to film formation, the substratewas subjected to degreasing washing by means of an ultrasonic wave for 5minutes in each of acetone, ethanol, and ultrapure water, and then wasdried in the air at 100° C.

A polycrystalline sintered material having an InGaO₃(ZnO)₄ composition(having a diameter of 20 mm and a thickness of 5 mm) was used for thetarget.

The sintered material was produced by wet-mixing 4N reagents of In₂O₃,Ga₂O₃, and ZnO as starting materials in ethanol as a solvent; calciningthe mixture at 1,000° C. for 2 hours; dry-pulverizing the resultant; andsintering the pulverized product at 1,550° C. for 2 hours. The target808 had an electric conductivity of 90 (S/cm), and was in asemi-insulating state.

The ultimate pressure in the growth chamber 821 was 1×10⁻⁴ (Pa) and thetotal pressure of an oxygen gas and the argon gas during growth wasmaintained at a constant value in the range of 4 to 0.1×10⁻¹ (Pa). Then,the ratio between the partial pressure of the argon gas and the oxygenpartial pressure was changed to change the oxygen partial pressure inthe range of 10⁻³ to 2×10⁻¹ (Pa).

In addition, the substrate temperature was set to be room temperature,and the distance between the target 808 and the deposition substrate 807was 30 (mm).

Supplied power was RF180 W, and film formation was performed at a filmforming rate of 10 (nm/min).

X-ray diffraction was conducted on the resultant film by means of anX-ray at an angle of incidence as close as the surface of the film (thinfilm method, angle of incidence 0.5 degree). As a result, no cleardiffraction peak was detected. Therefore, the produced In—Zn—Ga—O-basedfilm was found to be an amorphous film.

Furthermore, X-ray reflectance measurement was performed, and patternanalysis was performed. As a result, the thin film was found to have amean square roughness (Rrms) of about 0.5 nm and a thickness of about120 nm. X-ray fluorescence (XRF) analysis confirmed that the metalcomposition ratio of the thin film was In:Ga:Zn=0.98:1.02:4.

The oxygen partial pressure in the atmosphere upon film formation waschanged to measure the electric conductivity of the resultant amorphousoxide film. FIG. 3 shows the results.

As shown in FIG. 3, film formation in an atmosphere having a high oxygenpartial pressure in excess of 3×10⁻² Pa was able to reduce an electricconductivity to less than 10 S/cm.

Additionally increasing the oxygen partial pressure was able to reducethe number of electron carriers.

For example, as shown in FIG. 3, an InGaO₃(ZnO)₄ thin film formed at asubstrate temperature of 25° C. and an oxygen partial pressure of 10⁻¹Pa had an electric conductivity additionally reduced to about 10⁻¹⁰S/cm. In addition, an InGaO₃(ZnO)₄ thin film formed at an oxygen partialpressure in excess of 10⁻¹ Pa had so high an electrical resistance thatits electric conductivity could not be measured. In this case, theelectron mobility, which could not be measured, was estimated to beabout 1 cm²/V·sec as a result of extrapolation from a value in a filmhaving a large electron carrier concentration.

That is, it was able to constitute a normally-off transistor having anon-off ratio in excess of 10³ by using a transparent amorphous oxidethin film which is constituted of In—Ga—Zn—O produced by means of asputtering deposition method in an argon gas atmosphere having an oxygenpartial pressure in excess of 3×10⁻² Pa, or desirably in excess of5×10⁻¹ Pa, and has a composition in a crystalline state represented byInGaO₃(ZnO)_(m) (where m represents a natural number of less than 6).

When the apparatus and the material shown in this embodiment are used,the oxygen partial pressure upon film formation by means of sputteringis, for example, in the range of 3×10⁻² Pa to 5×10⁻¹ Pa (bothinclusive). As shown in FIG. 2, the electron mobility increases withincreasing number of conduction electrons in a thin film produced bymeans of each of the pulse laser deposition method and the sputteringmethod.

As described above, controlling an oxygen partial pressure can reducethe number of oxygen defects, thereby reducing an electron carrierconcentration. In addition, in an amorphous state, unlike apolycrystalline state, substantially no particle interface is present,so an amorphous thin film having a high electron mobility can beobtained.

It should be noted that an InGaO₃(ZnO)₄ amorphous oxide film obtained byusing a polyethylene terephthalate (PET) film having a thickness of200·m instead of a glass substrate also showed similar characteristics.

The use of a polycrystal InGaO₃(Zn_(1-x)Mg_(x)O)_(m) (where m representsa natural number of less than 6 and 0<x·1) as a target provides ahigh-resistance amorphous InGaO₃(Zn_(1-x)Mg_(x)O)_(m) film even at anoxygen partial pressure of less than 1 Pa.

For example, when a target obtained by replacing Zn with 80 at. % of Mgis used, the electron carrier concentration of a film obtained by meansof a pulse laser deposition method in an atmosphere having an oxygenpartial pressure of 0.8 Pa can be less than 10¹⁶/cm³ (the electricalresistance is about 10⁻² S/cm).

The electron mobility of such film reduces as compared to a film with noadditional Mg, but the degree of the reduction is small: the electronmobility at room temperature is about 5 cm²/(V·sec), which is about oneorder of magnitude larger than that of amorphous silicon. Upon filmformation under the same conditions, the electric conductivity and theelectron mobility reduce with increasing Mg content. Therefore, the Mgcontent is preferably in excess of 20% and less than 85% (that is,0.2<x<0.85).

In the thin film transistor using the above-described amorphous oxidefilm, one of Al₂O₃, Y₂O₃, and HfO₂, or a mixed crystal compoundcontaining at least two kinds of these compounds is preferably used fora gate insulation film.

When a defect is present in an interface between the gate insulationthin film and the channel layer thin film, an electron mobility reducesand hysteresis occurs in transistor characteristics. In addition, leakcurrent varies to a large extent depending on the kind of the gateinsulation film. Therefore, a gate insulation film suitable for achannel layer needs to be selected. The use of an Al₂O₃ film can reduceleak current. In addition, the use of a Y₂O₃ film can reduce hysteresis.Furthermore, the use of an HfO₂ film having a high dielectric constantcan increase electron mobility. In addition, the use of a mixed crystalof those films can result in the formation of a TFT having a small leakcurrent, small hysteresis, and large electron mobility. In addition,each of a gate insulation film forming process and a channel layerforming process can be performed at room temperature, so each of astaggered structure and an inversely staggered structure can be formedas a TFT structure.

The TFT thus formed is a three-terminal device equipped with a gateterminal, a source terminal, and a drain terminal, and is an activedevice which uses a semiconductor thin film formed on an insulatingsubstrate such as a ceramic, glass, or plastic as a channel layer inwhich an electron or a hole moves, and provides a switching function fora current between the source terminal and the drain terminal by applyinga voltage to the gate terminal to control a current flowing in thechannel layer.

The fact that a desired electron carrier concentration can be achievedby controlling an oxygen defective amount is important in the presentinvention.

In the foregoing description, the amount of oxygen in an amorphous oxidefilm (oxygen defective amount) is controlled in an atmosphere containinga predetermined concentration of oxygen upon film formation. It is alsopreferable to control (reduce or increase) the oxygen defective amountby subjecting the oxide film to a post treatment in an atmospherecontaining oxygen after the film formation.

To effectively control the oxygen defective amount, the temperature inthe atmosphere containing oxygen is in the range of desirably 0° C. to300° C. (both inclusive), preferably 25° C. to 250° C. (both inclusive),or more preferably 100° C. to 200° C. (both inclusive).

Of course, the oxygen defective amount may be controlled in theatmosphere containing oxygen upon film formation and then controlledthrough a post treatment in the atmosphere containing oxygen after thefilm formation. In addition, the oxygen partial pressure may becontrolled not upon film formation but after the film formation througha post treatment in the atmosphere containing oxygen as long as adesired electron carrier concentration (less than 10¹⁸/cm³) can beobtained.

The lower limit for the electron carrier concentration in the presentinvention, which varies depending on what kind of device, circuit, orapparatus an oxide film to be obtained is used for, is, for example,10¹⁴/cm³ or more.

(Expansion of Material System)

As a result of research on an expanded composition system, it has beenfound that an amorphous oxide film having a small electron carrierconcentration and a large electron mobility can be produced by means ofan amorphous oxide composed of an oxide of at least one element of Zn,In, and Sn.

It has also been found that the amorphous oxide film has a specificproperty with which the electron mobility increases with increasingnumber of conduction electrons.

A normally-off TFT excellent in transistor characteristics includingon-off ratio, saturation current in a pinch-off state, and switchingspeed can be produced by means of the film.

A composite oxide containing at least one element of the followingelements can be constituted by using the above-described amorphous oxidecontaining at least one element of Zn, In, and Sn.

The elements are a Group II element M2 having an atomic number smallerthan that of Zn (M2 represents Mg or Ca); a Group III element M3 havingan atomic number smaller than that of In (M3 represents B, Al, Ga, orY); a Group IV element M4 having an atomic number smaller than that ofSn (M4 represents Si, Ge, or Zr); a Group V element M5 (M5 represents V,Nb, or Ta); Lu; and W.

An oxide having any one of the following characteristics (a) to (h) canbe used in the present invention.

(a) An amorphous oxide having an electron carrier concentration of lessthan 10¹⁸/cm³ at room temperature.(b) An amorphous oxide whose electron mobility increases with increasingelectron carrier concentration.

The term “room temperature” as used herein refers to a temperature ofabout 0° C. to 40° C. The term “amorphous” as used herein refers to acompound having only a halo pattern to be observed, and showing nospecific diffraction ray, in an X-ray diffraction spectrum. The term“electron mobility” as used herein refers to an electron mobilitymeasured through Hall effect measurement.

(c) An amorphous oxide according to the above item (a) or (b) havingelectron mobility in excess of 0.1 cm²/V·sec at room temperature.(d) An amorphous oxide according to the above item (b) or (c) exhibitingdegenerate conduction. The term “degenerate conduction” as used hereinrefers to a state where heat activation energy in the temperaturedependence of an electrical resistance is 30 meV or less.(e) An amorphous oxide according any one of the above items (a) to (d)containing at least one element of Zn, In, and Sn as a constituent.(f) An amorphous oxide film obtained by incorporating, into theamorphous oxide according to the above item (e), at least one element ofa Group II element M2 having an atomic number smaller than that of Zn(M2 represents Mg or Ca); a Group III element M3 having an atomic numbersmaller than that of In (M3 represents B, Al, Ga, or Y); a Group IVelement M4 having an atomic number smaller than that of Sn (M4represents Si, Ge, or Zr); a Group V element M5 (M5 represents V, Nb, orTa); Lu; and W.(g) An amorphous oxide film according to any one of the above items (a)to (f), which is a single compound having a composition in a crystallinestate represented by In_(1-x)M3 _(x)O₃(Zn_(1-y)M2 _(y)O)_(m) (where 0·x,y·1 and m represents 0 or a natural number of less than 6) or a mixtureof compounds having different m's. M3 represents Ga or the like, and M2represents Mg or the like.(h) An amorphous oxide film according to any one of the above items (a)to (g) which is arranged on a glass substrate, a metal substrate, aplastic substrate, or a plastic film.

The present invention relates to a field effect transistor using theamorphous oxide or amorphous oxide film described above for a channellayer.

An amorphous oxide film having an electron carrier concentration inexcess of 10¹⁵/cm³ and less than 10¹⁸/cm³ is used for a channel layer toconstitute a field effect transistor in which a source terminal, a drainterminal, and a gate terminal are arranged via a gate insulation film.When a voltage of about 5 V is applied between the source and drainterminals, the current between the source and drain terminals with nogate voltage applied can be about 10⁻⁷ A.

The electron mobility of an oxide crystal increases as the degree towhich the s orbitals of metal ions overlap with each other increases.The oxide crystal of Zn, In, or Sn having a large atomic number has alarge electron mobility of 0.1 to 200 cm²/(V·sec).

Furthermore, in the oxide, oxygen and a metal ion bond to each otherthrough an ionic bond.

As a result, even in an amorphous state in which a chemical bond has nodirectivity, a structure is random, and the direction of bonding isnon-uniform, the electron mobility can be comparable to the electronmobility in a crystalline state.

On the other hand, replacing Zn, In, or Sn with an element having asmall atomic number reduces the electron mobility. As a result, theelectron mobility of the amorphous oxide according to the presentinvention is about 0.01 cm²/(V·sec) to 20 cm²/(V·sec).

When a channel layer of a transistor is produced by means of theabove-described oxide, one of Al₂O₃, Y₂O₃, and HfO₂, or a mixed crystalcompound containing at least two kinds of these compounds is preferablyused for a gate insulation film in the transistor.

When a defect is present in an interface between the gate insulationthin film and the channel layer thin film, electron mobility reduces andhysteresis occurs in transistor characteristics. In addition, leakcurrent varies to a large extent depending on the kind of the gateinsulation film. Therefore, a gate insulation film suitable for achannel layer needs to be selected. The use of an Al₂O₃ film can reduceleak current. In addition, the use of an Y₂O₃ film can reducehysteresis. Furthermore, the use of an HfO₂ film having a highdielectric constant can increase field effect mobility. In addition, theuse of a film composed of a mixed crystal of those compounds can resultin the formation of a TFT having small leak current, small hysteresis,and large field effect mobility. In addition, each of a gate insulationfilm forming process and a channel layer forming process can beperformed at room temperature, so each of a staggered structure and aninversely staggered structure can be formed as a TFT structure.

An In₂O₃ oxide film can be formed by means of a vapor phase method, andan amorphous film can be obtained by adding about 0.1 Pa of water to anatmosphere during film formation.

Although an amorphous film is hardly obtained from each of ZnO and SnO₂,an amorphous film can be obtained by adding about 20 at % of In₂O₃ toZnO or by adding about 90 at % of In₂O₃ to SnO₂. In particular, about0.1 Pa of nitrogen gas is desirably introduced into the atmosphere inorder to obtain an Sn—In—O-based amorphous oxide film.

The above amorphous oxide film can has an additional elementconstituting a composite oxide of at least one element of a Group IIelement M2 having an atomic number smaller than that of Zn (M2represents Mg or Ca); a Group III element M3 having an atomic numbersmaller than that of In (M3 represents B, Al, Ga, or Y); a Group IVelement M4 having an atomic number smaller than that of Sn (M4represents Si, Ge, or Zr); a Group V element M5 (M5 represents V, Nb, orTa); Lu; and W.

The additional element can additionally stabilize the amorphous film atroom temperature. In addition, the addition can expand the compositionrange in which the amorphous film can be obtained.

In particular, the addition of B, Si, or Ge having strong covalency iseffective in stabilizing an amorphous phase, and a composite oxidecomposed of ions different from each other in ionic radius to a largeextent has a stabilized amorphous phase.

For example, a stable amorphous oxide semiconductor film is hardlyobtained at room temperature unless In accounts for more than about 20at % of an In—Zn—O system. However, the addition of Mg in an amountequivalent to that of In can provide a stable amorphous oxide film whenIn accounts for more than about 15 at %.

An amorphous oxide semiconductor film having electron carrierconcentration in excess of 10¹⁵/cm³ and less than 10¹⁸/cm³ can beobtained by controlling an atmosphere in film formation by means of avapor phase method.

An amorphous oxide semiconductor is desirably formed into a film bymeans of any one of the vapor phase methods such as a pulse laserdeposition method (PLD method), a sputtering method (an SP method), andan electron beam deposition method. Of those vapor phase methods, a PLDmethod is suitable because the composition of a material system can beeasily controlled, and an SP method is suitable in terms of massproductivity. However, a film forming method is not limited to thosemethods.

(Formation of In—Zn—Ga—O-Based Amorphous Oxide Film by Means of PLDMethod)

Polycrystalline sintered materials each having an InGaO₃(ZnO)composition or an InGaO₃(ZnO)₄ composition were used as targets todeposit an In—Zn—Ga—O-based amorphous oxide film on a glass substrate(1737 manufactured by Corning Inc.) by means of a PLD method using a KrFexcimer laser.

The film forming apparatus used was that shown in FIG. 14 describedabove, and film forming conditions were the same as those in the casewhere the apparatus was used.

The substrate temperature was 25° C. X-ray diffraction was conducted oneach of the resultant films by means of small angle X-ray scatteringmethod (SAXS; thin film method, angle of incidence 0.5 degree). As aresult, no clear diffraction peak was detected. Therefore, each of theIn—Zn—Ga—O-based films produced from two kinds of targets was found tobe an amorphous film.

Furthermore, X-ray scattering measurement was performed on each of theIn—Zn—Ga—O-based films on the glass substrate, and pattern analysis wasperformed. As a result, each of the thin films was found to have a meansquare roughness (Rrms) of about 0.5 nm and a thickness of about 120 nm.

X-ray fluorescence (XRF) analysis confirmed that the metal compositionratio of the film obtained by using the polycrystalline sinteredmaterial having the InGaO₃(ZnO) composition as a target was In:GaZn=1.1:1.1:0.9 and the metal composition ratio of the film obtained byusing the polycrystalline sintered material having the InGaO(ZnO)₄composition as a target was In:Ga:Zn=0.98:1.02:4.

The electron carrier concentration of the amorphous oxide semiconductorfilm obtained by using the polycrystalline sintered material having theInGaO₃(ZnO)₄ composition as a target was measured with the oxygenpartial pressure of the atmosphere during film formation changed. FIG. 1shows the results. Film formation in an atmosphere having an oxygenpartial pressure in excess of 4.2 Pa was able to reduce the electroncarrier concentration to less than 10¹⁸/cm³. In this case, the substratehad a temperature maintained at a temperature nearly equal to roomtemperature unless intentionally heated. When the oxygen partialpressure was less than 6.5 Pa, the surface of the resultant amorphousoxide film was flat.

When the oxygen partial pressure was 5 Pa, the amorphous oxide filmobtained by using the polycrystalline sintered material having theInGaO₃(ZnO)₄ composition as a target had an electron carrierconcentration of 10¹⁶/cm³ and an electric conductivity of 10⁻² S/cm. Inaddition, its electron mobility was estimated to be about 5 cm²/V·sec.Owing to the analysis of a light absorption spectrum, the forbidden bandenergy width of the produced amorphous oxide film was determined to beabout 3 eV.

Additionally increasing the oxygen partial pressure was able toadditionally reduce the electron carrier concentration. As shown in FIG.1, an In—Zn—Ga—O-based amorphous oxide film formed at a substratetemperature of 25° C. and an oxygen partial pressure of 6 Pa had anelectron carrier concentration reduced to 8×10¹⁵/cm³ (electricconductivity: about 8×10⁻³ S/cm). The electron mobility of the resultantfilm was estimated to be in excess of 1 cm²/(V·sec). However, in the PLDmethod, when the oxygen partial pressure was 6.5 Pa or more, the surfaceof the deposited film became irregular, so it became difficult to usethe film as a channel layer of a TFT.

Investigation was made into the relationship between the electroncarrier concentration and electron mobility of each of In—Zn—Ga—O-basedamorphous oxide semiconductor films formed at different oxygen partialpressures by using the polycrystalline sintered material having theInGaO₃(ZnO)₄ composition as a target. FIG. 2 shows the results. It wasfound that the electron mobility increased from about 3 cm²/(V·sec) toabout 11 cm²/(V·sec) as the electron carrier concentration increasedfrom 10¹⁶/cm³ to 10²⁰/cm³. A similar tendency was observed in anamorphous oxide film obtained by using the polycrystalline sinteredmaterial having the InGaO₃(ZnO) composition as a target.

An In—Zn—Ga—O-based amorphous oxide semiconductor film obtained by usinga polyethylene terephthalate (PET) film having a thickness of 200·minstead of a glass substrate also showed similar characteristics.

(Formation of In—Zn—Ga—Mg—O-Based Amorphous Oxide Film by Means of PLDMethod)

A polycrystal InGaO₃(Zn_(1-x)Mg_(x)O)₄ (0<x·1) was used as a target toform an InGaO₃(Zn_(1-x)Mg_(x)O)₄ (0<x·1) film on a glass substrate bymeans of the PLD method.

The apparatus shown in FIG. 14 was used as a film forming apparatus.

An SiO₂ glass substrate (1737 manufactured by Corning Inc.) was preparedas a deposition substrate. The substrate was subjected to degreasingwashing by means of an ultrasonic wave for 5 minutes in each of acetone,ethanol, and ultrapure water as a pretreatment, and then was dried inthe air at 100° C. An InGa(Zn_(1-x)Mg_(x)O)₄ (x=1 to 0) sinteredmaterial (having a diameter of 20 mm and a thickness of 5 mm) was usedas a target.

The target was produced by wet-mixing 4N reagents of In₂O₃, Ga₂O₃, ZnO,and MgO as starting materials in ethanol as a solvent; calcining themixture at 1,000° C. for 2 hours; dry-pulverizing the resultant; andsintering the pulverized product at 1,550° C. for 2 hours. The ultimatepressure in the growth chamber was 2×10⁻⁶ (Pa), and an oxygen partialpressure during growth was set to be 0.8 (Pa). The substrate temperaturewas room temperature (25° C.), and the distance between the target andthe deposition substrate was 30 (mm).

The KrF excimer laser had a power of 1.5 (mJ/cm²/pulse), a pulse widthof 20 (nsec), a pulse rate of 10 (Hz), and an irradiation spot diameterof 1×1 (mm square).

The film-forming rate was 7 (nm/min).

The oxygen partial pressure of the atmosphere was 0.8 Pa, and thesubstrate temperature was 25° C. X-ray diffraction was conducted on theresultant film by means of small angle X-ray scattering method (SAXS;thin film method, angle of incidence 0.5 degree). As a result, no cleardiffraction peak was detected. Therefore, the producedIn—Zn—Ga—Mg—O-based film was found to be an amorphous film. The surfaceof the resultant film was flat.

Targets having different values of x were used to determine the x valuedependence of each of the electric conductivity, electron carrierconcentration, and electron mobility of the In—Zn—Ga—Mg—O-basedamorphous oxide film formed in an atmosphere having an oxygen partialpressure of 0.8 Pa.

FIG. 4 shows the results. When the value of x exceeded 0.4, the electroncarrier concentration of an amorphous oxide film formed by means of thePLD method in an atmosphere having an oxygen partial pressure of 0.8 Pawas found to be less than 10¹⁸/cm³. In addition, an amorphous oxide filmhaving a value of x in excess of 0.4 had an electron mobility in excessof 1 cm²/V·sec.

As shown in FIG. 4, when a target obtained by replacing Zn with 80 at. %of Mg is used, the electron carrier concentration of a film obtained bymeans of a pulse laser deposition method in an atmosphere having anoxygen partial pressure of 0.8 Pa can be less than 10¹⁶/cm³ (theelectrical resistance is about 10⁻² S/cm). The electron mobility of suchfilm reduces as compared to a film with no additional Mg, but the degreeof the reduction is small: the electron mobility at room temperature isabout 5 cm²/(V·sec), which is about one order of magnitude larger thanthat of amorphous silicon. Upon film formation under the sameconditions, the electric conductivity and the electron mobility reducewith increasing Mg content. Therefore, the Mg content is preferably inexcess of 20 at % and less than 85 at % (that is, 0.2<x<0.85), morepreferably 0.5<x<0.85.

An InGaO₃(Zn_(1-x)Mg_(x)O)₄ (0<x·1) amorphous oxide film obtained byusing a polyethylene terephthalate (PET) film having a thickness of200·m instead of a glass substrate also showed similar characteristics.

(Formation of In₂O₃ Amorphous Oxide Film by Means of PLD Method)

An In₂O₃ polycrystalline sintered material was used as a target to forman In₂O₃ film on a PET film having a thickness of 200·m by means of thePLD method using a KrF excimer laser.

The apparatus shown in FIG. 14 was used. An SiO₂ glass substrate (1737manufactured by Corning Inc.) was prepared as a deposition substrate.

The substrate was subjected to degreasing washing by means of anultrasonic wave for 5 minutes in each of acetone, ethanol, and ultrapurewater as a pretreatment, and then was dried in the air at 100° C.

An In₂O₃ sintered material (having a diameter of 20 mm and a thicknessof 5 mm) was used as a target. The target was prepared by calcining a 4Nreagent of In₂O₃ as a starting material at 1,000° C. for 2 hours;dry-pulverizing the resultant; and sintering the pulverized product at1,550° C. for 2 hours.

The ultimate pressure in the growth chamber was 2×10⁻⁶ (Pa), and theoxygen partial pressure during growth and the substrate temperature wereset to be 5 (Pa) and room temperature, respectively.

The oxygen partial pressure and the vapor partial pressure were set tobe 5 Pa and 0.1 Pa, respectively, and 200 W was applied to anoxygen-radical-generating apparatus to generate an oxygen radical.

The distance between the target and the deposition substrate was 40(mm). The KrF excimer laser had a power of 0.5 (mJ/cm²/pulse), a pulsewidth of 20 (nsec), a pulse rate of 10 (Hz), and an irradiation spotdiameter of 1×1 (mm square).

The film forming rate was 3 (nm/min).

X-ray diffraction was conducted on the resultant film by means of anX-ray at an angle of incidence as close as the surface of the film (thinfilm method, angle of incidence 0.5 degree). As a result, no cleardiffraction peak was detected. Therefore, the produced In—O-based filmwas found to be an amorphous film. The film had a thickness of 80 nm.

The resultant In—O-base amorphous oxide film had an electron carrierconcentration of 5×10¹⁷/cm³ and an electron mobility of about 7cm²/V·sec.

(Formation of In—Sn—O-Based Amorphous Oxide Film by Means of PLD Method)

An (In_(0.9)Sn_(0.1))O_(3.1) polycrystalline sintered material was usedas a target to form an In—Sn—O-based oxide film on a PET film having athickness of 200·m by means of the PLD method using a KrF excimer laser.

To be specific, an SiO₂ glass substrate (1737 manufactured by CorningInc.) was prepared as a deposition substrate.

The substrate was subjected to degreasing washing by means of anultrasonic wave for 5 minutes in each of acetone, ethanol, and ultrapurewater as a pretreatment. After that, the substrate was then dried in theair at 100° C.

An In₂O₃—SnO₂ sintered material (having a diameter of 20 mm and athickness of 5 mm) was prepared as a target. The target was produced bywet-mixing a 4N reagent of In₂O₃—SnO₂ as a starting material in ethanolas a solvent; calcining the mixture at 1,000° C. for 2 hours;dry-pulverizing the resultant; and sintering the pulverized product at1,550° C. for 2 hours.

The substrate temperature was room temperature. The oxygen partialpressure and the nitrogen partial pressure were set to be 5 (Pa) and 0.1(Pa), respectively, and 200 W was applied to anoxygen-radical-generating apparatus to generate oxygen radical.

The distance between the target and the deposition substrate was 30(mm). The KrF excimer laser had a power of 1.5 (mJ/cm²/pulse), a pulsewidth of 20 (nsec), a pulse rate of 10 (Hz), and an irradiation spotdiameter of 1×1 (mm square).

The film-forming rate was 6 (nm/min).

X-ray diffraction was conducted on the resultant film by means of smallangle scattering method (SAXA; thin film method, angle of incidence 0.5degree). As a result, no clear diffraction peak was detected. Therefore,the produced In—Sn—O-based film was found to be an amorphous film.

The resultant In—Sn—O amorphous oxide film had an electron carrierconcentration of 8×10¹⁷/cm³, an electron mobility of about 5 cm²/V·sec,and a thickness of 100 nm.

(Formation of In—Ga—O-Based Amorphous Oxide Film by Means of PLD Method)

An SiO₂ glass substrate (1737 manufactured by Corning Inc.) was preparedas a deposition substrate.

The substrate was subjected to degreasing washing by means of anultrasonic wave for 5 minutes in each of acetone, ethanol, and ultrapurewater as a pretreatment, and then was dried in the air at 100° C.

An (In₂O₃)_(1-x)—(Ga₂O₃)_(x) (x=0 to 1) sintered material (having adiameter of 20 mm and a thickness of 5 mm) was prepared as a target. Forexample, in the case of x=0.1, the target is a (In_(0.9)Ga_(0.1))₂O₃polycrystalline sintered material.

The target was produced by wet-mixing a 4N reagent of In₂O₃—Ga₂O₂ as astarting material in ethanol as a solvent; calcining the mixture at1,000° C. for 2 hours; dry-pulverizing the resultant; and sintering thepulverized product at 1,550° C. for 2 hours.

The ultimate pressure in the growth chamber was 2×10⁻⁶ (Pa), and theoxygen partial pressure during growth was set to be 1 (Pa).

The substrate temperature was room temperature. The distance between thetarget and the deposition substrate was 30 (mm). The KrF excimer laserhad a power of 1.5 (mJ/cm²/pulse), a pulse width of 20 (nsec), a pulserate of 10 (Hz), and an irradiation spot diameter of 1×1 (mm square).The film-forming rate was 6 (nm/min).

The substrate temperature was 25° C. The oxygen partial pressure was 1Pa. X-ray diffraction was conducted on the resultant film by means of anX-ray at an angle of incidence as close as the surface of the film (thinfilm method, angle of incidence 0.5 degree). As a result, no cleardiffraction peak was detected. Therefore, the produced InGa—O-based filmwas found to be an amorphous film. The film had a thickness of 120 nm.

The resultant In—Ga—O amorphous oxide film had an electron carrierconcentration of 8×10¹⁶/cm³ and an electron mobility of about 1cm²/V·sec.

(Production of TFT Device Using In—Zn—Ga—O-Based Amorphous Oxide Film(Glass Substrate)) Production of TFT Device

A top gate TFT device shown in FIG. 5 was produced.

At first, a polycrystalline sintered material having an InGaO₃(ZnO)₄composition was used as a target to form an In—Ga—Zn—O-based amorphousoxide film on a glass substrate 1 at an oxygen partial pressure of 5 Paby means of the above-described PLD apparatus. Thus, an In—Ga—Zn—O-basedamorphous film having a thickness of 120 nm to be used as a channellayer 2 was formed.

An In—Ga—Zn—O-based amorphous film and a gold film each having a largeelectric conductivity and a thickness of 30 nm were laminated on thefilm by means of the PLD method with the oxygen partial pressure in thechamber set to be less than 1 Pa, to thereby form a drain terminal 5 anda source terminal 6 by means of a photolithography method and a lift-offmethod.

Finally, a Y₂O₃ film to be used as a gate insulation film 3 (thickness:90 nm, relative dielectric constant: about 15, leak current density:10⁻³ A/cm² upon application of 0.5 MV/cm) was formed by means of anelectron beam deposition method. A gold film was formed on the Y₂O₃film, to thereby form a gate terminal 4 by means of a photolithographymethod and a lift-off method. The channel length was 50·m and thechannel width was 200·m.

Evaluation of TFT Device for Characteristics

FIG. 6 shows the current-voltage characteristics of a TFT devicemeasured at room temperature. The fact that a drain current I_(DS)increased with increasing drain voltage V_(DS) shows that the conductionof the channel is of an n-type.

This is not in contradiction to the fact that an amorphousIn—Ga—Zn—O-based amorphous oxide film is an n-type conductor. I_(DS)saturated (pinched off) at V_(DS) of about 6 V. The saturation is atypical behavior of a semiconductor transistor. Investigation into again characteristic showed that the threshold value for a gate voltageV_(GS) was about −0.5 V upon application of V_(DS)=4 V.

A current I_(DS)=1.0×10⁻⁵ A flowed when V_(G)=10 V. This corresponds tothe fact that a gate bias enabled a carrier to be induced in anIn—Ga—Zn—O-based amorphous oxide film as an insulator.

The transistor had an on-off ratio in excess of 10³. The field effectmobility was calculated from an output characteristic. As a result, afield effect mobility of about 7 cm²(Vs)⁻¹ was obtained in thesaturation region. The produced device was irradiated with visible lightto perform similar measurement. However, no changes in transistorcharacteristics were observed.

An amorphous oxide having an electron carrier concentration of less than10¹⁸/cm³ is applicable to a channel layer of a TFT. The electron carrierconcentration was more preferably 10¹⁷/cm³ or less, or still morepreferably 10¹⁶/cm³ or less.

(Production of TFT Device Using In—Zn—Ga—O-Based Amorphous Oxide Film(Amorphous Substrate))

A top gate TFT device shown in FIG. 5 was produced. At first, apolycrystalline sintered material having an InGaO₃(ZnO) composition wasused as a target to form an In—Zn—Ga—O-based amorphous oxide film havinga thickness of 120 nm to be used as a channel layer 2 on a polyethyleneterephthalate (PET) film 1 at an oxygen partial pressure of 5 Pa bymeans of the PLD method.

An In—Zn—Ga—O-based amorphous oxide film and a gold film each having alarge electric conductivity and a thickness of 30 nm were laminated onthe film by means of the PLD method with the oxygen partial pressure inthe chamber set to be less than 1 Pa, to thereby form a drain terminal 5and a source terminal 6 by means of a photolithography method and alift-off method. Finally, a gate insulation film 3 was formed by meansof an electron beam deposition method, and a gold film was formed on thefilm to thereby form a gate terminal 4 by means of a photolithographymethod and a lift-off method. The channel length was 50·m and thechannel width was 200·m. Each of Y₂O₃ (thickness: 140 nm), Al₂O₃(thickness: 130·m), and HfO₂ (thickness: 140·m) was used as a gateinsulation film to produce three kinds of TFT's each having the abovestructure.

Evaluation of TFT Device for Characteristics

The current-voltage characteristics of a TFT formed on the PET filmmeasured at room temperature were the same as those shown in FIG. 6.That is, the fact that a drain current I_(DS) increased with increasingdrain voltage V_(DS) shows that the conduction of the channel is of ann-type. This is not in contradiction to the fact that an amorphousIn—Ga—Zn—O-based amorphous oxide film is an n-type conductor. I_(DS)saturated (pinched off) at V_(DS) of about 6 V. The saturation is atypical behavior of a transistor. A current I_(ds)=10⁻⁸ A flowed whenV_(g)=0, while a current I_(DS)=2.0×10⁻⁵ A flowed when V_(g)=10 V. Thiscorresponds to the fact that a gate bias enabled an electron carrier tobe induced in an In—Ga—Zn—O-based amorphous oxide film as an insulator.

The transistor had an on-off ratio in excess of 10³. The field effectmobility was calculated from an output characteristic. As a result, afield effect mobility of about 7 cm²(Vs)⁻¹ was obtained in a saturationregion.

The device produced on the PET film was bent at a radius of curvature of30 mm to perform similar measurement of transistor characteristics.However, no changes in transistor characteristics were observed. Thedevice was irradiated with visible light to perform similar measurement.However, no changes in transistor characteristics were observed.

The TFT using an Al₂O₃ film as a gate insulation film showed transistorcharacteristics similar to those shown in FIG. 6. A current I_(ds)=10⁻⁸A flowed when V_(g)=0, while a current I_(DS)=5.0×10⁻⁶ A flowed whenV_(g)=10 V. The transistor had an on-off ratio in excess of 10². Thefield effect mobility was calculated from an output characteristic. As aresult, a field effect mobility of about 2 cm² (Vs)⁻¹ was obtained in asaturation region.

The TFT using an HfO₂ film as a gate insulation film showed transistorcharacteristics similar to those shown in FIG. 6. A current I_(ds)=10⁻⁸A flowed when V_(g)=0, while a current I_(DS)=1.0×10⁻⁶ A flowed whenV_(g)=10 V. The transistor had an on-off ratio in excess of 10². Thefield effect mobility was calculated from an output characteristic. As aresult, a field effect mobility of about 10 cm²(Vs)⁻¹ was obtained in asaturation region.

(Production of TFT Device Using In₂O₃ Amorphous Oxide Film by Means ofPLD Method)

A top gate TFT device shown in FIG. 5 was produced. At first, an In₂O₃amorphous oxide film having a thickness of 80 nm to be used as a channellayer 2 was formed on a polyethylene terephthalate (PET) film 1 by meansof the PLD method.

Then, an In₂O₃ amorphous oxide film and a gold film each having a largeelectric conductivity and a thickness of 30 nm were laminated on thefilm by means of the PLD method with the oxygen partial pressure in thechamber set to be less than 1 Pa and a voltage to be applied to anoxygen-radical-generating apparatus set to zero, to thereby form a drainterminal 5 and a source terminal 6 by means of a photolithography methodand a lift-off method. Finally, a Y₂O₃ film to be used as a gateinsulation film 3 was formed by means of an electron beam depositionmethod, and a gold film was formed on the film to thereby form a gateterminal 4 by means of a photolithography method and a lift-off method.

Evaluation of TFT Device for Characteristics

The current-voltage characteristics of the TFT formed on the PET filmwere measured at room temperature. The fact that a drain current I_(DS)increased with increasing drain voltage V_(DS) shows that the channel isan n-type semiconductor. This is not in contradiction to the fact thatan InO-based amorphous oxide film is an n-type conductor. I_(DS)saturated (pinched off) at V_(DS) of about 5 V. The saturation is atypical behavior of a transistor. A current I_(DS)=2×10⁻⁸ A flowed whenV=0 V, while a current I^(DS)=2.0×10⁻⁶ A flowed when V_(G)=10 V. Thiscorresponds to the fact that a gate bias enabled an electron carrier tobe induced in an InO-based amorphous oxide film as an insulator.

The transistor had an on-off ratio of about 10². The field effectmobility was calculated from an output characteristic. As a result, afield effect mobility of about 10 cm²(Vs)⁻¹ was obtained in a saturationregion. A TFT device produced on a glass substrate showed similarcharacteristics.

The device produced on the PET film was bent at a radius of curvature of30 mm to perform similar measurement of transistor characteristics.However, no changes in transistor characteristics were observed.

(Production of TFT device using In—Sn—O-Based Amorphous Oxide Film byMeans of PLD Method)

A top gate TFT device shown in FIG. 5 was produced. At first, anIn—Sn—O-based amorphous oxide film having a thickness of 100 nm to beused as a channel layer 2 was formed on a polyethylene terephthalate(PET) film 1 by means of the PLD method. Then, an In—Sn—O-basedamorphous oxide film and a gold film each having a large electricconductivity and a thickness of 30 nm were laminated on the film bymeans of the PLD method with the oxygen partial pressure in the chamberset to be less than 1 Pa and a voltage to be applied to anoxygen-radical-generating apparatus set to zero, to thereby form a drainterminal 5 and a source terminal 6 by means of a photolithography methodand a lift-off method. Finally, a Y₂O₃ film to be used as a gateinsulation film 3 was formed by means of an electron beam depositionmethod, and a gold film was formed on the film to thereby form a gateterminal 4 by means of a photolithography method and a lift-off method.

Evaluation of TFT Device for Characteristics

The current-voltage characteristics of the TFT formed on the PET filmwere measured at room temperature. The fact that a drain current I_(DS)increased with increasing drain voltage V_(DS) shows that the channel isan n-type semiconductor. This is not in contradiction to the fact thatan In—Sn—O-based amorphous oxide film is an n-type conductor. I_(DS)saturated (pinched off) at V_(DS) of about 6 V. The saturation is atypical behavior of a transistor. A current I_(DS)=5×10⁻⁸ A flowed whenV_(g)=10 V, while a current I_(DS)=5.0×10⁻⁵ A flowed when V_(d)=10 V.This corresponds to the fact that a gate bias enabled an electroncarrier to be induced in an In—Sn—O-based amorphous oxide film as aninsulator.

The transistor had an on-off ratio of about 10³. The field effectmobility was calculated from an output characteristic. As a result, afield effect mobility of about 5 cm²(Vs)⁻¹ was obtained in a saturationregion. A TFT device produced on a glass substrate showed similarcharacteristics.

The device produced on the PET film was bent at a radius of curvature of30 mm to perform similar measurement of transistor characteristics.However, no changes in transistor characteristics were observed.

(Production of TFT Device Using In—Ga—O-Based Amorphous Oxide Film byMeans of PLD Method)

A top gate TFT device shown in FIG. 5 was produced. At first, anIn—Ga—O-based amorphous oxide film having a thickness of 120 nm to beused as a channel layer 2 was formed on a polyethylene terephthalate(PET) film 1 by means of a film forming method shown in Example 6. Then,an In—Ga—O-based amorphous oxide film and a gold film each having alarge electric conductivity and a thickness of 30 nm were laminated onthe film by means of the PLD method with the oxygen partial pressure inthe chamber set to be less than 1 Pa and the voltage to be applied to anoxygen-radical-generating apparatus set to zero, to thereby form a drainterminal 5 and a source terminal 6 by means of a photolithography methodand a lift-off method. Finally, a Y₂O₃ film to be used as a gateinsulation film 3 was formed by means of an electron beam depositionmethod, and a gold film was formed on the film to thereby form a gateterminal 4 by means of a photolithography method and a lift-off method.

Evaluation of TFT Device for Characteristics

The current-voltage characteristics of the TFT formed on the PET filmwere measured at room temperature. The fact that a drain current I_(DS)increased with increasing drain voltage V_(DS) shows that the channel isan n-type semiconductor. This is not in contradiction to the fact thatan In—Ga—O-based amorphous oxide film is an n-type conductor. I_(DS)saturated (pinched off) at V_(DS) of about 6 V. The saturation is atypical behavior of a transistor. A current I_(DS)=1×10⁻⁸ A flowed whenV_(g)=0 V, while a current I_(DS)=1.0×10⁻⁶ A flowed when V_(G)=10 V.This corresponds to the fact that a gate bias enabled an electroncarrier to be induced in an In—Ga—O-based amorphous oxide film as aninsulator.

The transistor had an on-off ratio of about 10². A field effect mobilitywas calculated from an output characteristic. As a result, a fieldeffect mobility of about 0.8 cm²(Vs)⁻¹ was obtained in a saturationregion. A TFT device produced on a glass substrate showed similarcharacteristics.

The device produced on the PET film was bent at a radius of curvature of30 mm to perform similar measurement of transistor characteristics.However, no changes in transistor characteristics were observed.

An amorphous oxide semiconductor having an electron carrierconcentration of less than 10¹⁸/cm³ is applicable to a channel layer ofa TFT. The electron carrier concentration was more preferably 10¹⁷/cm³or less, or still more preferably 10¹⁶/cm³ or less.

FIG. 7 shows the transmittance of an amorphous oxide semiconductor layer(200 nm in thickness) constituted of In—Ga—Zn—O and having a compositionin a crystalline state represented by InGaO₃(Zn)_(m) (where m representsa natural number of less than 6). The band gap is about 3 eV. This layerhas a strong sensitivity particularly to the ultraviolet light that hasa wavelength shorter than 400 nm and has a transmission of 60% or less.An amorphous oxide semiconductor layer constituted of In—Ga—Zn—Mg—O andhaving a composition in a crystalline state represented byInGaO₃(Zn_(1-x)Mg_(x)O)_(m) (where m represents a natural number of lessthan 6 and 0<x·1) shows a similar transmittance, and shows sensitivityto ultraviolet light.

In addition, the use of an organic pigment can expand the lightsensitivity wavelength range of an amorphous oxide semiconductor mainlycomposed of In—Ga—Zn—O having a large electron mobility from anultraviolet wavelength range to a visible light wavelength range, andcauses the semiconductor to show high opto-electric conversionefficiency.

Hereinafter, examples will be shown.

Example 1

An optical sensor device shown in FIG. 8 is formed. An Al electrodehaving a thickness of 100 nm is formed on a glass substrate (1737manufactured by Corning Inc.) by means of a vacuum deposition method toserve as a lower electrode. Next, a polycrystalline sintered materialhaving an InGaO₃(ZnO)₄ composition is used as a target to deposit anIn—Ga—Zn—O-based amorphous oxide semiconductor thin film on theelectrode by means of a pulse laser deposition method using a Kr excimerlaser. In₂O₃(SnO₂) having a thickness of about 20 nm is laminated on thethin film at a substrate temperature of room temperature by means of avacuum deposition method to serve as an upper electrode. Thus, anoptical sensor is formed. At the time of use, a negative bias is appliedto the upper electrode on a light incidence side, and a positive bias isapplied to the lower electrode. Then, the optical sensor device isirradiated with ultraviolet light having a wavelength of 365 nm from amercury lamp. Thus, it can be confirmed that the device functions as anultraviolet optical sensor.

Example 2

An optical sensor device shown in FIG. 8 is formed. An Al electrodehaving a thickness of 100 nm is formed on a glass substrate (1737manufactured by Corning Inc.) by means of a vacuum deposition method toserve as a lower electrode. Next, a polycrystalline sintered materialhaving an InGaO₃(Zn_(0.9)Mg_(0.1)O)₄ composition is used as a target todeposit an In—Ga—Zn—O-based amorphous oxide semiconductor thin film onthe electrode by means of a pulse laser deposition method using a Krexcimer laser. The thickness of the amorphous oxide semiconductor filmformed is 100 nm. In₂O₃(SnO₂) having a thickness of about 20 nm islaminated on the thin film at a substrate temperature of roomtemperature by means of a vacuum deposition method to serve as an upperelectrode. A voltage of 1 V is applied to the optical sensor device thusformed. At the time of use, a negative bias is applied to the upperelectrode on a light incidence side, and a positive bias is applied tothe lower electrode. Then, the optical sensor device is irradiated withultraviolet light having a wavelength of 365 nm from a mercury lamp.Thus, it can be confirmed that the device functions as an ultravioletoptical sensor.

Example 3

An optical sensor device shown in FIG. 9 is formed. An Al electrodehaving a thickness of 100 nm is formed on a glass substrate (1737manufactured by Corning Inc.) by means of a vacuum deposition method toserve as a lower electrode. A polycrystalline sintered material havingan InGaO₃(Zn_(0.9)Mg_(0.1)O)₄ composition is used as a target to depositan In—Ga—Zn—O-based amorphous oxide semiconductor thin film having athickness of 5 nm on the electrode by means of a pulse laser depositionmethod using a Kr excimer laser. Next, a polycrystalline sinteredmaterial having an InGaO₃(ZnO)₄ composition is used as a target todeposit an In—Ga—Zn—O-based amorphous oxide semiconductor thin filmhaving a thickness of 5 nm. This operation is repeated 20 times tolaminate a semiconductor layer having a multilayer structure (200 nm inthickness). The total thickness of the amorphous oxide semiconductorfilms formed is 100 nm. In₂O₃(SnO₂) having a thickness of about 20 nm islaminated on the semiconductor layer at a substrate temperature of roomtemperature by means of a vacuum deposition method to serve as an upperelectrode.

Example 4

In the optical sensor device shown in Example 1, the amorphous oxidesemiconductor having a thickness of 100 nm is laminated, and is thenimmersed in a pigment solution prepared by dissolving 0.01% of a cyaninepigment into a mixed solution of methanol and chloroform to cause theorganic pigment to adsorb and bond to the semiconductor. After theorganic solvent has been volatilized, In₂O₃(SnO₂) having a thickness ofabout 20 nm is laminated on the remainder at a substrate temperature ofroom temperature by means of a vacuum deposition method to serve as anupper electrode. A voltage of 1 V is applied to the optical sensordevice thus formed. A negative bias is applied to the upper electrode ona light incidence side, and a positive bias is applied to the lowerelectrode. Then, the optical sensor device is irradiated withultraviolet light having a wavelength of 365 nm from a mercury lamp.Thus, it can be confirmed that the device functions as an ultravioletoptical sensor.

Example 5

An optical sensor device shown in FIG. 9 is formed. An Al electrodehaving a thickness of 100 nm is formed on a glass substrate (1737manufactured by Corning Inc.) by means of a vacuum deposition method toserve as a lower electrode. A polycrystalline sintered material havingan InGaO₃(Zn_(0.9)Mg_(0.1)O)₄ composition is used as a target to depositan In—Ga—Zn—O-based amorphous oxide semiconductor thin film having athickness of 5 nm on the electrode by means of a pulse laser depositionmethod using a Kr excimer laser. Next, a polycrystalline sinteredmaterial having an InGaO₃(ZnO)₄ composition is used as a target todeposit an In—Ga—Zn—O-based amorphous oxide semiconductor thin filmhaving a thickness of 5 nm. This operation is repeated 20 times tolaminate a semiconductor layer having a multilayer structure (200 nm inthickness). A voltage of 1 V is applied to the optical sensor devicethus formed. A negative bias is applied to the upper electrode on alight incidence side, and a positive bias is applied to the lowerelectrode. Then, the optical sensor device is irradiated withultraviolet light having a wavelength of 365 nm from a mercury lamp.Thus, it can be confirmed that the device functions as an ultravioletoptical sensor.

Example 6

A top gate MISFET device shown in FIG. 5 is produced as a TFT of anon-flat imager.

A polyimide sheet having a thickness of 0.3 mm is used as a substrate.

A polycrystalline sintered material having an InGaO₃(ZnO)₄ compositionis used as a target to deposit an In—Ga—Zn—O-based amorphous oxidesemiconductor thin film on the polyimide sheet by means of a pulse laserdeposition method using a KrF excimer laser. Thus, an InGaO₃(ZnO)₄amorphous oxide semiconductor thin film having a thickness of 120 nm tobe used as a channel layer is formed. Furthermore, InGaO₃(ZnO)₄ and agold film each having a large electric conductivity and a thickness of30 nm are laminated on the film by means of a pulse laser depositionmethod with the oxygen partial pressure in a chamber set to be less than1 Pa, to thereby form a drain terminal and a source terminal by means ofa photolithography method and a lift-off method. Finally, a Y₂O₃ film tobe used as a gate insulation film (thickness: 90 nm, relative dielectricconstant: about 15, leak current density: 10⁻³ A/cm upon application of0.5 MV/cm) is formed by means of an electron beam deposition method. Agold film is formed on the Y₂O₃ film, to thereby form a gate terminal bymeans of a photolithography method and a lift-off method.

An optical sensor device shown in FIG. 8 is formed as a sensor of anon-flat imager. An Al electrode having a thickness of 100 nm is formedon the polyimide substrate by means of a vacuum deposition method toserve as a lower electrode. Next, a polycrystalline sintered materialhaving an InGaO₃(ZnO)₄ composition is used as a target to deposit anIn—Ga—Zn—O-based amorphous oxide semiconductor thin film on theelectrode by means of a pulse laser deposition method using a Kr excimerlaser. The thickness of the amorphous oxide semiconductor film formed is100 nm. The amorphous oxide semiconductor is immersed in a pigmentsolution prepared by dissolving 0.01% of a cyanine pigment into a mixedsolution of methanol and chloroform to cause the organic pigment toadsorb and bond to the semiconductor.

In₂O₃(SnO₂) having a thickness of about 20 nm is laminated on the thinfilm at a substrate temperature of room temperature by means of a vacuumdeposition method to serve as an upper electrode. A CdWO₄ layer having athickness of 400·m to serve as a scintillator is deposited on the upperelectrode by means of a sputtering method. Then, an X-ray sensor shownin FIG. 10 is formed. Such TFT and X-ray sensor are combined to form acircuit shown in FIG. 11, thereby constituting a non-flat imager shownin FIG. 12. A small digital camera to serve a measuring object is placedin such non-flat imager to perform X-ray measurement. An image havingreduced distortion as compared to that of an image obtained by using aconventional flat X-ray imager can be obtained.

Example 7

A top gate MISFET device shown in FIG. 5 is produced as a TFT of anon-flat imager. A plastic sheet having a thickness of 0.3 mm is used asa substrate. A polycrystalline sintered material having anInGaO₃(Zn_(0.9)Mg_(0.1)O)₄ composition is used as a target to deposit anIn—Ga—Zn—O-based amorphous oxide semiconductor thin film on the plasticsheet by means of a pulse laser deposition method using a KrF excimerlaser. Thus, an InGaO₃(Zn_(0.9)Mg_(0.1)O)₄ amorphous oxide semiconductorthin film having a thickness of 120 nm to be used as a channel layer isformed. Furthermore, InGaO₃(Zn_(0.9)Mg_(0.1)O)₄ and a gold film eachhaving a large electric conductivity and a thickness of 30 nm arelaminated on the film by means of a pulse laser deposition method withthe oxygen partial pressure in the chamber set to be less than 1 Pa, tothereby form a drain terminal and a source terminal by means of aphotolithography method and a lift-off method. Finally, a Y₂O₃ film tobe used as a gate insulation film (thickness: 90 nm, relative dielectricconstant: about 15, leak current density: 10⁻³ A/cm upon application of0.5 MV/cm) is formed by means of an electron beam deposition method. Agold film is formed on the Y₂O₃ film, to thereby form a gate terminal bymeans of a photolithography method and a lift-off method.

An optical sensor device shown in FIG. 9 is formed as a sensor of anon-flat imager. An Al electrode having a thickness of 100 nm is formedon the plastic substrate by means of a vacuum deposition method to serveas a lower electrode. A polycrystalline sintered material having anInGaO₃(Zn_(0.9)Mg_(0.1)O)₄ composition is used as a target to deposit anIn—Ga—Zn—O-based amorphous oxide semiconductor thin film having athickness of 5 nm on the electrode by means of a pulse laser depositionmethod using a Kr excimer laser. Next, a polycrystalline sinteredmaterial having an InGaO₃(ZnO)₄ composition is used as a target todeposit an In—Ga—Zn—O-based amorphous oxide semiconductor thin filmhaving a thickness of 5 nm. This operation is repeated 20 times tolaminate a semiconductor layer having a multilayer structure (200 nm inthickness). The total thickness of the amorphous oxide semiconductorfilms formed is 100 nm. Every time each oxide semiconductor layer islaminated, a phthalocyanine pigment is vacuum-deposited to laminateabout a monomolecular film of the pigment on the oxide semiconductorlayer. In₂O₃(SnO₂) having a thickness of about 20 nm is laminated on theresultant at a substrate temperature of room temperature by means of avacuum deposition method to serve as an upper electrode.

Such TFT and X-ray sensor are combined to form a circuit shown in FIG.11, thereby constituting a non-flat imager shown in FIG. 12. A smalldigital camera to serve a measuring object is placed in such non-flatimager to perform X-ray measurement. An image having reduced distortionas compared to that of an image obtained by using a conventional flatX-ray imager can be obtained.

The present invention is applicable to a sensor and a non-flat imagereach having high sensitivity to ultraviolet light, visible light, and anX-ray.

This application claims priority from Japanese Patent Application No.2004-326681 filed Nov. 10, 2004, which is hereby incorporated byreference herein.

1-10. (canceled)
 11. An image pickup device comprising: a flexiblesubstrate; an X-ray sensor arranged on the flexible substrate; and afield effect transistor for reading a signal from the X-ray sensor,wherein: the field effect transistor has an amorphous oxide as an activelayer; and the amorphous oxide comprises one of an oxide having anelectron carrier concentration of less than 10¹⁸/cm³ and an oxide whoseelectron mobility tends to increase with increasing electron carrierconcentration.
 12. An image pickup device according to claim 11, whereina current between drain and source terminals of the field effecttransistor when no gate voltage is applied is less than 10 μA.
 13. Animage pickup device according to claim 12, further comprising a non-flatimaging region.
 14. (canceled)
 15. An image pickup device according toclaim 12, wherein the X-ray sensor includes a scintillator forconverting X-ray into light and an opto-electric conversion element. 16.An image pickup device comprising: a substrate having a non-flat region;an X-ray sensor provided on the substrate; and a field effect transistorfor reading a signal from the X-ray sensor, wherein the field effecttransistor is a normally-off transistor having an active layer composedof an amorphous oxide.
 17. An image pickup device according to claim 15,wherein a current between drain and source terminals of the field effecttransistor when no gate voltage is applied is less than 10 μA.
 18. Animage pickup device comprising: a sensor for detecting anelectromagnetic wave; and a field effect transistor for reading a signalfrom the sensor, wherein the field effect transistor has an amorphousoxide semiconductor as an active layer; and the amorphous oxidesemiconductor layer is composed of an oxide of a metal material selectedfrom at least one of In, Zn and Sn.
 19. An image pickup device accordingto claim 18, wherein a current between drain and source terminals of thefield effect transistor when no gate voltage is applied is less than 10μA.
 20. An image pickup device according to claim 19, wherein theamorphous oxide semiconductor layer comprises an oxide selected from thegroup consisting of an oxide containing In, Zn and Sn, an oxidecontaining In and Zn, an oxide containing In and Sn, and an oxidecontaining In.
 21. An image pickup device according to claim 19, whereinthe amorphous oxide semiconductor layer contains In, Ga, and Zn.
 22. Animage pickup device according to claim 19, wherein the sensor is anX-ray sensor, and the X-ray sensor includes a scintillator forconverting X-ray into light, and the sensor detects the light as theelectromagnetic wave.
 23. An image pickup device comprising: asubstrate; an X-ray sensor arranged on the substrate; and a field effecttransistor for reading a signal from the X-ray sensor, wherein: thefield effect transistor has an amorphous oxide as an active layer; andthe amorphous oxide comprises one of an oxide having an electron carrierconcentration of less than 1018/cm3 and an oxide whose electron mobilitytends to increase with increasing electron carrier concentration.
 24. Animage pickup device according to claim 23, wherein a current betweendrain and source terminals of the field effect transistor when no gatevoltage is applied is less than 10 μA.
 25. An image pickup devicecomprising: a substrate; an X-ray sensor provided on the substrate; anda field effect transistor for reading a signal from the X-ray sensor,wherein the field effect transistor is a normally-off transistor havingan active layer composed of an amorphous oxide.
 26. An image pickupdevice according to claim 25, wherein a current between drain and sourceterminals of the field effect transistor when no gate voltage is appliedis less than 10 μA.